1. Field of the Invention
The present invention relates to an information reader. The information reader is utilized as, for instance, a bar code reader. The bar code reader uses a hologram disk for reading the bar code of a bar code label attached to goods to be monitored.
2. Description of the Related Art
Conventional overhead type bar code readers incorporate a scanning unit placed above a path of movement of the goods. The scanning unit has a light source, a scanning system, and a sensing system. The light source emits a scanning light beam downward to read the bar code. In the overhead-type bar code reader, it is desirable to shorten the optical path length in the scanning unit so as to minimize the size of the scanning unit.
Prior Work
To shorten the optical path length, the overhead-type bar code reader described in U.S. patent application Ser. No. 562,865 filed on Aug. 6, 1990, now U.S. Pat. No. 5,153,417, (corresponding to Japanese Patent Application No. Heisei 1-203648) was proposed. In the disclosed bar code reader, a light blocking tube is provided in a condenser lens which collects the laser beam reflected by the bar code label. The light blocking tube extends through the condenser lens along the central optical axis of the condenser lens. A laser beam emitted from a light source is reflected by a reflecting mirror provided on the light blocking tube in a direction to advance along the longitudinal axis of the light blocking tube. The reflected laser beam impinges on a hologram disk at a right angle. The optical path length in the scanning unit of this bar code reader is shortened because the condenser lens as a light-collecting means is disposed near the hologram disk.
However, in the above-mentioned technique, the reflecting mirror and the light source are separately installed in the scanning unit. Therefore, it is easy to misalign the optical axes of the reflecting mirror and the light source. The incidence angle of the reflected laser beam on the hologram disk deviates from perpendicular.
In such a bar code reader, the incidence angle of a light beam on the hologram disk need to be adjusted with high accuracy. This is because the incidence angle has a large effect on the diffraction efficiency of the hologram disk. For instance, when the incidence angle is an unsuitable angle, optical intensity emitted from the hologram disk becomes weak. In such a case, the above-mentioned bar code reader can not read the bar code information accurately.
Moreover, when the optical axes are adjusted relative to each other after assembling, it is very difficult to adjust both of the position of the light blocking tube providing the reflecting mirror and the position of the light source (including the members for fixing these). This is because, after one of these is adjusted, the other need to be adjusted again.